Container closure caps now in widespread commercial use are constructed of metallic cap shells made of a suitable metal plate such as a tin plate, a tin-free steel sheet or an aluminum-base alloy sheet. A liner of a suitable plastic material such as a polyolefin resin (e.g. polyethylene or polypropylene) or a vinyl chloride resin is applied to the top panel of the caps at their inner surfaces.
The lining material is applied to the inner surface of the top panel of a metallic cap shell by apparatus which comprises a conveyor mechanism for conveying cap shells at a predetermined speed through a predetermined passage from a cap shell supply station to a cap shell transfer station through a lining material dispensing station. The apparatus also comprises a supply mechanism for supplying cap shells to the conveyor mechanism at the shell supply position, a heater disposed along the conveying passage between the shell supply station and the lining material dispensing station, a lining material dispensing mechanism for dispensing a predetermined amount of lining material into the cap shells at the lining material dispensing station, and a molding mechanism for receiving the cap shells from the conveyor mechanism at the shell transfer station and molding the lining material in the cap shells into a predetermined shape. Such an apparatus is described, for example, in the specifications and drawings of Japanese Patent Publication No. 20759/67, Japanese Laid-Open Patent Publication No. 99186/77 and Japanese Patent Application No. 82888/78.
A cap shell (especially its top panel) is heated in such an apparatus to a predetermined temperature by the heater, and thereafter, a lining material is dispensed into the heated cap shell and molded therein. Generally, the inside surface of the cap shell is coated with an adhesive primer composed, for example, of an epoxy-phenol type paint and oxidized polyethylene or maleinized polyethylene dispersed therein. Thus, when the cap shell is heated by the heater, the adhesive primer layer is also heated. Since the lining material is molded while the adhesive primer layer is at a high temperature, the molded lining material is bonded to the inner surface of the top panel of the cap shell by the adhesive primer.
Extensive investigations and experiments concerning the bond strength between the cap shell and the lining material as a function of heating of the cap shell have led to the discovery of certain facts. In the prior art apparatus described above, the cap shell is heated only between the cap shell supply station and the lining material dispensing station, and therefore, the cap shell is not heated at a position downstream of the lining material dispensing station. However, to mold the lining material dispensed into the cap shell, it is necessary to transfer the cap shell containing the dispensed lining material to the molding mechanism. Because of the nature of the structure of the mechanism for moving the cap shell, it is necessary to transfer the cap shell from the lining material dispensing station to the molding station through a passage of a predetermined length. It has been found in the aforementioned apparatus that when the cap shell is composed of a metallic material having high thermal conductivity such as an aluminum base alloy, the temperature of the cap shell decreases considerably due to heat dissipation during movement of the shell from the lining material dispensing station to the molding mechanism. Consequently, the temperature drop tends to reduce the bond strength between the cap shell and the molded lining material. If an attempt is made to prevent temperature drop by heating the cap shell to a sufficiently high temperature before it reaches the lining material dispensing station, then it is necessary to heat the cap shell rapidly during its travel from the shell supply station to the lining material dispensing station. However, it is extremely difficult to heat the top panel of the cap shell uniformly by rapid heating, and as a result, the temperature distribution of the top panel of the shell becoms non-uniform. In particular, in an apparatus where cap shells are conveyed at a high speed, this non-uniformity in temperature distribution is quite apparent.
In order to solve this problem of uneven heating of the top panel of a cap shell associated with prior art apparatus, it has been proposed to heat cap shells between the lining material dispensing station and the shell transfer station instead of heating the shell between the shell supply station and the lining material dispensing station. If this alternative method of heating is employed, a cap shell is not heated at the lining material dispensing station but is at a low temperature. Accordingly, when a plastic lining material is dispensed from an extruder into a predetermined position on the inner surface of the top panel of a cap shell, the lining material is displaced with respect to the cap shell by inertia and cannot be properly held in position on the inner surface of the top panel thus posing a further problem. In addition, the conveying passage between the lining material dispensing station and the cap shell transfer station must be made longer than is necessary in order to heat the cap shell to a desired high temperature sufficiently and uniformly.
It is therefore an object of this invention to provide an an apparatus for dispensing and molding lining material into metallic cap shells where the molded lining material will be securely bonded to the inner surfaces of the cap shells with sufficient bond strengths while overcoming the problems mentioned above.